A SCUBA-2 850-$\mu$m survey of circumstellar disks in the $\lambda$ Orionis cluster

Abstract

We present results from an 850-$\mu$m survey of the $\sim$ 5 Myr old $\lambda$ Orionis star-forming region. We used the SCUBA-2 camera on the James Clerk Maxwell Telescope to survey a $\sim$0.5-diameter circular region containing 36 (out of 59) cluster members with infrared excesses indicative of circumstellar disks. We detected only one object at $>3\sigma$ significance, the Herbig Ae star HD 245185, with a flux density of $\sim 74$ mJy beam${}^{-1}$ corresponding to a dust mass of $\sim 150$ M${}_{\oplus }$. Stacking the individually undetected sources did not produce a significant mean signal but gives an upper limit on the average dust mass for $\lambda$ Orionis disks of $\sim 3$ M${}_{\oplus }$. Our follow-up observations of HD 245185 with the Submillimeter Array found weak CO 2--1 line emission with an integrated flux of $\sim 170$ mJy km s${}^{-1}$ but no ${}^{13}$CO or C${}^{18}$O isotopologue emission at 30 mJy km s${}^{-1}$ sensitivity, suggesting a gas mass of $\lesssim 1$ M${}_{\mathrm{Jup}}$. The implied gas-to-dust ratio is thus $\gtrsim 50$ times lower than the canonical interstellar medium value, setting HD 245185 apart from other Herbig Ae disks of similar age, which have been found to be gas rich; as HD 245185 also shows signs of accretion, we may be catching it in the final phases of disk clearing. Our study of the $\lambda$ Orionis cluster places quantitative constraints on planet formation timescales, indicating that at $\sim 5$ Myr the average disk no longer has sufficient dust and gas to form giant planets and perhaps even super Earths; the bulk material has been mostly dispersed or is locked in pebbles/planetesimals larger than a few mm in size.

Accepted to ApJ (23 April 2015), 10 pages, 6 figures